Sandbox25

'''This sandbox is in use until August 1, 2011 for UMass Chemistry 423. Others please do not edit this page. Thanks!

Chem423 Team Projects: Understanding Drug Mechanisms'''

Group Members: Varun Chalupadi, Anthony Laviola, Tiffany Brucker, Alan Stebbins

Introduction
Cyclooxygenase, abbreviated COX, is an enzyme involved in the formation of biological mediators and takes part in the pain and inflammatory response. It serves as an effective pain and inflammation signal in the body to indicate a fault in the body’s homeostatic balance. Drugs target the binding sites of COX to prevent substrate binding and therefore reduce pain and inflammation in the body.

There are two commonly used forms of cyclooxygenase in animals which are denoted as COX-1 and COX-2. COX-1 carries out normal, physiological production of prostaglandins and serves as a basic housekeeping messages throughout the body. Alternatively, COX-2 is constructed in specialty cells and is used in pain and inflammation signaling. COX-2 is “induced by cytokines, mitogens and endotoxins in inflammatory cells, and which is responsible for the production of prostaglandins in inflammation.”

COX has been of research interest because of the value it provides in particular signaling pathways. Currently, cyclooxygenase is widely targeted in the production of a group of drugs called non-steroidal anti-inflammatory drugs (NSAIDS). This group of drugs inclue asprin, ibuprofecn, flurbiprofen and acetaminophen. These drugs attack the binding site of cyclooxygenase and prevent the substrate binding to reduce pain and fever. NSAIDS are broken down into four different classes. Asprin is categorized in class one, ibuprofem is in class two, flurbiprofen and indomethacin are examples of class three, and Vioxx and Celebrex are components of class four. Latest research shows that cyclooxygenase can possibly serve as an effective target in battling cancer including lung and bladder cancer. Additionally, research is being conducted to evaluate the effectiveness of targeting COX in studying Alzhemier’s disease and cardiovascular disease.

Structurally, cyclooxygenase is composed primarily of alpha helices with few beta sheets. The protein contains two binding sites, the cyclooxygenase active site and the peroxidase site.

Overall Structure
COX-2 is a homodimer membrane protein with two identical subunits. The secondary structure of each subunit contains primarily alpha helices, shown in light blue, with a few beta sheets, shown in yellow. Each subunit contains 587 amino acids.

Each subunit contains three domains, the epidermal growth factor (red) beginning at the N-terminus, followed by a membrane binding domain (green), and a large catalytic domain at the C-terminus which contains 480 amino acids (blue). The catalytic domain contains two active sites, the cyclooxygenase and peroxidase. The membrane binding domain is made up of four alpha helices. The alpha helices are amphipathic, creating a hydrophobic surface, shown in gray, which integrates into the membrane bilayer.

COX-1 and COX-2 are very conserved, being 67% identical in their amino acid sequences. The greatest difference occurs in the membrane binding domain which is only 33% identical.

Drug Binding Site
Many drugs such as aspirin, tylenol, and ibuprofen help regulate pain and and the inflammatory response in the body by blocking the active site of COX. These drugs along with others not only inhibit COX-2 but also inhibit COX-1, causing severe side effects in a small percentage of patients. Recently there has been advances in selectively regulating COX-2 without affecting COX-1 with a drug such as Vioxx. The enzyme COX-2 breaks down arachidonic acid to initiate the production of prostaglandins. Arachidonic acid enters the enzyme through a hydrophobic tunnel formed by the four alpha-helices in the second domain. Once the arachidonic acid reaches the peroxidase active site, a hydrogen is believed to be ripped off by the tyrosine 385. The heme group located near the peroxidase active site which is believed to help stabilize the radical formed. This radical goes on to further processes that create prostaglandins which intensify pain signals and induce inflammation in damaged parts of the body.

The COX enzyme can be regulated by four different classes of non-steroidal anti-inflammatory drugs (NSAIDS). The first class of inhibitors such as Aspirin regulate the enzyme by irreversibly inactivating the enzyme through covalent modification. A second class of NSAIDS like Motrin or Advil competitively regulates the enzyme. The third class of NSAIDS, for example Flurbiprofen and Indomethacin, forms salt bridges with the enzyme resulting in a slow, time dependent regulation. Finally the fourth class of drugs namely Vioxx and Celebrex selectively regulates COX-2. Aspirin inhibits the COX enzyme by acetylating the serine residue in the catalytic site preventing the substrate from being catalyzed. Flurbiprofen, a class three inhibitor, binds to the hydrophobic tunnel preventing the substrate from reaching the active site. Flurbiprofen does this with a number of interactions. First it binds to the 120 arginine residue with the formation of a salt bridge. It then hydrogen bonds with the 355 tyrosine residue. An example of a class four inhibitor is SC-558. This drug works in a very similar way as the class three drugs in that it blocks the hydrophobic tunnel but it selectively affects the COX-2. Due to an exposed pocket in COX-2 that is not accessible in COX-1, this drug selectively regulates COX-2 19,000 times more effectively then COX-1. The phenylsulphonamide group of SC-558 binds to this pocket.

Additional Features
Role in other conditions and diseases

COX-2 does not only aid in pain response but plays a role in numerous conditions and diseases including Alzheimer's disease, cardiovascular disease, and cancers such as of the lung and bladder.

Recent discoveries have shown that COX-2 has indirectly played a role in smoker related cancers. Cigarette smoke has been proven to decrease COX-2 expression and cause an increase of PGE2 and TxA2 release. This imbalance causes the progression of tumors and carcinogenesis. This imbalance also contributed to progression of cardiovascular disease. Particularly, more COX-2 positive tumors were found in lung cancer patients than COX-1 positive tumors. Furthermore, smokers of non-cancerous and cancerous patients both had higher expression and imbalance of COX-2, PGE2, and TxA2. Thus, COXIBs are a possible candidate for research in some possible anti-tumor reagents.

Overexpression of COX-2 is observed in later and severe stages of Alzheimer's disease. COX-2 is found to be expressed normally in brain neurons; however, it is questioned as to whether COX-2 prevents or causes neuronal cell death. Some research indicates that an increase in COX-2 and PG synthesis may cause neuronal cell death. Other research has found that NSAID treatment in rats with Alzheimer's disease decreased activated microglial cells and could be used to treat Alzheimer's disease in small doses. Further research must be clarified about the role of COX-2 in the hippocampus, as its role is not clearly defined.

NSAIDS have been shown to have adverse side effects on the kidneys and cause fluid retention, hypertension, edema, and hyperkalemia through the alterations of renal blood flow, and sodium and potassium excretion. COX-2 has selective inhibition and is expressed in the kidneys, thus, with NSAID use severe side effects can occur.

Credits
Introduction: Varun Chalupadi

Overall Structure: Alan Stebbins

Drug Binding Site: Anthony Laviola

Additional Features: Tiffany Brucker

References (edited by): Tiffany Brucker